Malignant plasma cells secrete a number of effector and regulator products; among these are immunoglobulin, osteoclast activating factor, interferon and PC factor. PC factor appears to act upon macrophages, inducing them to secrete a plasmacytoma-induced macrophage substance (PIMS) which suppresses antibody production by antigen-stimulated normal spleen cells. This regulatory loop is responsible for the profound immunosuppression observed in hosts of plasma cell tumors. PC factor is produced and secreted into the surrounding medium by malignant plasma cells. Studies are proposed to isolate, purify and characterize the PC factor. Studies will be conducted to determine the type of macrophages which are stimulated by PC factor to produce PIMS, to determine the type of macrophages which are stimulated by PC factor to produce PIMS and to determine if the signal transmitted by PC factor can be mimicked by any other tumor-produced factors or macrophage-stimulating agents. Experiments will also be performed to determine if PC factor is produced by normal, non-malignant plasma cells or by mitogen-\or antigen-stimulated B lymphocytes.